1. Field of the Invention
The present general inventive concept relates to an aspheric lens that can be used as an objective lens in an optical information storage media system recording/reproducing information to/from an information storage medium, and to an optical pickup apparatus including the aspheric lens as an objective lens.
2. Description of the Related Art
Generally, an aspheric single objective lens is used to focus light onto an information storage medium in an optical information storage media system.
The most important factor in determining the storage capacity of an optical information storage medium, e.g., an optical disc, is the size of a light spot. When the size of a light spot is smaller, the size of marks or pits that can be recorded/reproduced is also smaller. Thus, the storage density of an optical disc can be increased.
To reduce the size of a light spot, a light source emitting a short wavelength light, such as blue laser, and an objective lens with a high numerical aperture (NA) are commonly used. For example, according to the Blue-ray disc (BD) standard, a blue light with an wavelength of approximately 405 nm and an objective lens with a NA of 0.85 are employed.
However, as the wavelength of light becomes shorter and the NA value of an objective lens becomes higher, an optical system employing the same becomes more vulnerable to various aberrations.
Therefore, when an aspheric single objective lens with a high NA is used for high density information storage media, such aberrations are compensated for by aspheric lens surfaces.
An aspheric surface equation generally used for designing an aspheric lens is as follows.
                              Z          ⁡                      (            r            )                          =                                            cr              2                                      1              +                                                1                  -                                                            (                                              1                        +                        K                                            )                                        ⁢                                          c                      2                                        ⁢                                          r                      2                                                                                                    +                      Ar            4                    +                      Br            6                    +                      Cr            8                    +                      Dr            10                    +                      Er            12                    +                      Fr            14                    +                      Gr            16                    +                      Hr            18                    +                      Jr            20                                              [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1,
      cr    2        1    +                  1        -                              (                          1              +              K                        )                    ⁢                      c            2                    ⁢                      r            2                              indicates quadric surfaces such as spherical surfaces or elliptical surfaces, and the high degree (higher-order) terms from r4 to r20 are applied for correcting aberrations. As shown in Equation 1, the generally high terms from r4 to r20 are used in the case of an aspheric lens.
In case of an objective lens for BD, the NA is as high as 0.85. Thus, it is necessary to correct aberrations also in the periphery of the lens, which explains the use of high degree terms when designing the shape of an aspheric lens.
However, when high degree terms are used for designing a lens, variation of an aspheric surface becomes larger. Although this large variation significantly contributes to correct aberrations, the angle of inclination on the surface of the lens also becomes larger. In this case, the permitted axial deviation of the surfaces of the lens becomes smaller.